Re: [REGRESSION] commit 66c1ca0: {fbmem: fix fb_info->lock and mm->mmap_sem ...} causes Xfbdev not working

[Andrea Righi <righi.andrea@gmail.com>]

On Fri, Apr 10, 2009 at 10:21:22PM +0200, Krzysztof Helt wrote:
> On Thu, 9 Apr 2009 14:58:51 +0200
> Andrea Righi <righi.andrea@gmail.com> wrote:
> 
> > On Thu, Apr 09, 2009 at 07:36:24PM +0800, Eric Miao wrote:
> > > This happens on my Marvell PXA310-based Littleton platform with
> > > Angstrom Distribution. The offending paths are many:
> > > 
> > > FBIOPUT_VSCREENINFO:
> > >   lock_fb_info()
> > >   --> fb_set_var()
> > >     --> fb_notifier_call_chain() [FBINFO_MISC_USEREVENT]
> > >       --> fbcon_event_notifier() [FB_EVENT_MODE_CHANGE]
> > >         --> lock_fb_info()
> > > 
> > > OK, now hang. I'd suggest a clean fix to the original assumption of
> > > circular locking
> > > issue and revert this commit first.
> > > 
> > > -- 
> > > Cheers
> > > - eric
> > 
> > I can agree to revert 66c1ca019078220dc1bf968f2bb18421100ef147, since I
> > don't have a clean fix for this. Pushing down fb_info->lock in
> > fb_set_var() excluding to call fb_notifier_call_chain with fb_info->lock
> > held doesn't seem to be so trivial...
> > 
> > However, reverting this will re-introduce the circular locking
> > dependency fb_info->lock => mm->mmap_sem => fb_info->lock.
> > 
> 
> If anybody is interested I have looked into the code of the fb_mmap()
> and have found that the problem Andrea tried to fix is caused
> only by drivers which implements their own fb_mmap().

Krzysztof,

first of all thanks for looking into the code!

> 
> The correct solution is to push the info->lock mutex into
> the fb_mmap() implemented inside the drivers. Some drivers
> may not need it (the ones which only calculate offsets
> and do not "real" iommaping).

mmmh... I may have missed something, but the common fb_mmap() should
acquire mm->mmap_sem and then info->lock, while fb_ioctl() can do that
in reverse order (info->lock first and then mm->mmap_sem) causing the
circular locking dependency. Are you sure that pushing info->lock down
each driver's fb_mmap will fix the problem?

> 
> I'll try to prepare a patch after the Easters. This will require a revert
>  of the 66c1ca commit.

Thanks and happy Easter,
-Andrea

> 
> Happy Easters,
> Krzysztof
> 
> ----------------------------------------------------------------------
> Nagrody w jednym miejscu!
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>